In many emerging wireless standards that support broadband data, Orthogonal Frequency Division Multiplexing (OFDM) modulation is commonly used with high order modulation formats, such as 16-QAM or 64-QAM. This modulation format is used due to its high spectral efficiency and tolerance to channel fading. To support these high order modulation formats, the channel should have relatively high fidelity, with an error vector magnitude (EVM) around 2% or lower. Because of this, severe constraints are placed on the linearity of the power amplifiers both at the network base stations or access points and at the customer terminals, or subscriber stations.
To overcome this problem, a conventional approach involves backing off the average output power of the power amplifier a significant margin from its compression point. However, as the back-off increases, the efficiency of the power amplifier decreases, resulting in a reduction in terminal battery life.
In addition, for OFDM waveforms, the peak-to-average power ratio (PAPR) is relatively high such that even further back-off is needed. Thus, the EVM requirements coupled with the high PAPR characteristics of OFDM waveforms result in a power amplifier with very low efficiency both at the terminal and at the base station or access point. As a result, battery drain is increased and power amplifier thermal dissipation uses more die area and increases cost. Therefore, there is a need in the art for a method of improving the efficiency of the power amplifier, especially when using OFDM waveforms, while maintaining or improving the waveform EVM and adjacent channel power ratio.